Influenza viruses are made up of an internal ribonucleoprotein core containing a segmented single-stranded RNA genome and an outer lipoprotein envelope lined by a matrix protein. Influenza A and B viruses each contain eight segments of single stranded RNA with negative polarity. The influenza A genome encodes at least eleven polypeptides. Segments 1-3 encode the three polypeptides, making up the viral RNA-dependent RNA polymerase. Segment 1 encodes the polymerase complex protein PB2. The remaining polymerase proteins PB1 and PA are encoded by segment 2 and segment 3, respectively. In addition, segment 1 of some influenza A strains encodes a small protein, PB1-F2, produced from an alternative reading frame within the PB1 coding region. Segment 4 encodes the hemagglutinin (HA) surface glycoprotein involved in cell attachment and entry during infection. Segment 5 encodes the nucleocapsid nucleoprotein (NP) polypeptide, the major structural component associated with viral RNA. Segment 6 encodes a neuraminidase (NA) envelope glycoprotein. Segment 7 encodes two matrix proteins, designated M1 and M2, which are translated from differentially spliced mRNAs. Segment 8 encodes NS1 and NS2 (NEP), two nonstructural proteins, which are translated from alternatively spliced mRNA variants.
The eight genome segments of influenza B encode 11 proteins. The three largest genes code for components of the RNA polymerase, PB1, PB2 and PA. Segment 4 encodes the HA protein. Segment 5 encodes NP. Segment 6 encodes the NA protein and the NB protein. Both proteins, NB and NA, are translated from overlapping reading frames of a biscistronic mRNA. Segment 7 of influenza B also encodes two proteins: M1 and BM2. The smallest segment encodes two products: NS1 is translated from the full length RNA, while NS2 is translated from a spliced mRNA variant.
Vaccines capable of producing a protective immune response specific for influenza viruses have been produced for over 50 years. Vaccines can be characterized as whole virus vaccines, split virus vaccines, surface antigen vaccines and live attenuated virus vaccines. While appropriate formulations of any of these vaccine types is able to produce a systemic immune response, live attenuated virus vaccines are also able to stimulate local mucosal immunity in the respiratory tract.
To date, all commercially available influenza vaccines in the United States have been propagated in embryonated hens' eggs. Although influenza virus generally grows well in hens' eggs, some influenza vaccine strains, such as the prototype A/Fujian/411/02 strain that circulated during the 2003-04 season, do not replicate well in embryonated hens' eggs, and have to be isolated by cell culture in a costly and time consuming procedure.
The ability of certain influenza virus strains to replicate to high titer in embryonated hens' eggs has been mapped to the M1 and M2 genes. See Klimov et al., 1991, Virus Res. 19:105-114. However, these studies identified only a single residue in the M1 gene that correlates with increased viral titer. Accordingly, identification of additional M1 residues associated with increased viral titer is needed to permit design and construction of recombinant and/or reassortant influenza viruses with increased replication capacity. These and other unmet needs are provided by the present invention.